Insulating foam core panels commonly include a core of expanded rigid foam material, and the core is laminated to facing sheets which may be metal, plastics, wood or paper. The facing sheets add strength to the assembled foam core panel and also protect the foam core from damage by moisture or impact by an object. One method of producing rigid foam core panels is the foam-in-place method wherein liquid plastics compounds are blended with blowing agents within the space or cavity defined between rigid facing sheets held in parallel spaced positions. The foam expands to fill the space and to adhere to the facing sheets while the foam cures to form a rigid core. Another method of producing foam core panels is to laminate the facing sheets by adhesive to opposite sides of preformed rigid foam panels.
It is usually desirable that the foam core material have small fine cells which are generally uniform in size and shape and are uniformally dispersed throughout the foam core with the volume of gases enclosed within the cells being approximately 20 times the volume of the foam plastics material forming the walls of the cells. The smaller the cell size, the better the thermal insulating properties of the rigid foam material. However, a small cell foam is more difficult to produce with the desired uniformity of cell size, shape, wall thickness and dispersement than foam having larger cells. Small cell foam boards are produced by extruding the foam material into a board having a thickness of no greater than three inches. When it is desired to produce foam core panels having a thickness greater than three inches, it is common to use planks of foam material expanded within a mold and having substantially larger cells with lower insulating properties.